CVE-2026-107301: CWE-1321: Improperly Controlled Modification of Object Prototype Attributes ('Prototype Pollution') in mcollina msgpack5
Description
### Impact Passing an empty or partial options object disables the default `protoAction: 'error'` protection. A map containing a `__proto__` key can then replace the prototype of the decoded object, potentially changing inherited properties or causing unexpected behavior in downstream code. Only the decoded object's prototype is affected; this does not modify `Object.prototype` globally. ### Patches Options are now merged with secure defaults without modifying the caller's object. Unsupported `protoAction` values are rejected. ### Workarounds Explicitly set `protoAction: 'error'` whenever constructing a msgpack5 instance, and validate decoded values before use.
CVSS v3.1
Score 6.5medium
Affected software
mcollina
msgpack5
Run on your own infrastructure? Check whether these packages are installed with threat-finder — our free open-source scanner.
Weaknesses
AI-Powered Analysis
Machine-generated threat intelligence
Technical Analysis
msgpack5, a MessagePack v5 implementation for Node.js and browsers, prior to version 6.1.0, improperly controls modification of object prototype attributes when constructed with an empty or partial options object. This disables the default protoAction: 'error' protection, enabling an attacker to supply a decoded map with a __proto__ key that replaces the decoded object's prototype. This can change inherited properties or downstream behavior without globally modifying Object.prototype. The vulnerability is classified as CWE-1321 and has a CVSS 3.1 score of 6.5 (medium severity). The issue is resolved in msgpack5 version 6.1.0.
Potential Impact
An attacker can manipulate the prototype of decoded objects, potentially altering inherited properties or causing unexpected behavior in downstream code that relies on these objects. This does not affect the global Object.prototype but can lead to integrity issues in application logic that uses msgpack5 for decoding. The vulnerability has no impact on confidentiality but impacts integrity and has low impact on availability.
Mitigation Recommendations
Upgrade to msgpack5 version 6.1.0 or later, where this prototype pollution vulnerability is fixed. Avoid constructing msgpack5 instances with empty or partial options objects that disable the default protoAction: 'error' protection. No other mitigation is indicated.
Technical Details
- Data Version
- 5.2
- Assigner Short Name
- GitHub_M
- Date Reserved
- 2026-10-07T15:53:23.587Z
- Cvss Version
- 3.1
- State
- PUBLISHED
Threat ID: 6ac7d1292cdf04f6562ba9b3
Added to database: 10/08/2026, 17:21:45 UTC
Last enriched: 10/08/2026, 17:33:35 UTC
Last updated: 10/08/2026, 21:48:49 UTC
Views: 7
Community Reviews
0 reviewsCrowdsource mitigation strategies, share intel context, and vote on the most helpful responses. Sign in to add your voice and help keep defenders ahead.
Want to contribute mitigation steps or threat intel context? Sign in or create an account to join the community discussion.
Actions
Updates to AI analysis require Pro Console access. Upgrade inside Console → Billing.
Need more coverage?
Upgrade to Pro Console for AI refresh and higher limits.
For incident response and remediation, OffSeq services can help resolve threats faster.
Latest Threats
Check if your credentials are on the dark web
Instant breach scanning across billions of leaked records. Free tier available.